Abstract The activating NKG2D receptor expressed on T cells and NK cells recognizes a polygenic and polymorphic family of ligands with structural homology to major histocompatibility complex class I proteins. These ligands are not expressed or are expressed in only low amounts by healthy tissues of adults, but they are frequently over- expressed by tumors, are up-regulated after infection with viruses and bacteria, and have been detected in certain autoimmune diseased tissues. Although the NKG2D ligands are typically expressed on the cell surface as transmembrane-anchored or glycosylphosphatidylinositol (GPI)-anchored protein, some of these ligands, including MICA, MICB, and ULBP-2, can either be secreted or shed from tumor cells. These soluble NKG2D ligands are frequently detected in the sera of human cancer patients, leading to the hypothesis that they may allow tumors to escape NKG2D-mediated immune responses by serving as decoy ligands for NKG2D. The overall goal of this project is to determine the consequences of NKG2D ligand expression in autoimmune diseases and in innate and adaptive immune responses against cancer and infectious diseases. To meet this goal, we will develop new mouse models that will help us and other investigators to understand the functions of NKG2D ligands and how these functions can be regulated to relieve disease. In aim 1, we will establish mice in which a cell surface NKG2D ligand, Rae-1, can be selectively expressed in any cell type or tissue of interest. Initially, we will use these mice to express Rae-1 exclusively on islet cells in the pancreas to determine the impact on the development of autoimmunity and, separately, on the development of primary pancreatic tumors. In aim 2, these mice will be used to express Rae-1 on dendritic cells (DC) to explore how this impacts the cross- talk between DC, NK cells, and T cells. In aim 3, we will express a soluble NKG2D ligand systemically in transgenic mice or in a tumor cell-specific manner to formally address whether soluble NKG2Dligands can allow immune evasion by tumors and if soluble NKG2D ligands impair immune defense against pathogens. Collectively, these studies will provide new insights into the role of NKG2D and its ligands in autoimmunity and in host defense, and they will provide new model systems for the pre-clinical evaluation of therapeutics targeting the NKG2D pathway for treatment of autoimmunity and cancer.